Floor leveler under partition wall?

Greetings all, first off a big thank you to all the people that responded to my fireplace demo thread. I have removed the hearth and scraped up all the loose mortar and such. I'm having the chimney cleaned and buttoned up and after that (and cleaning/up reworking the sexy temporary attic blocking pieces of wood) it's on to building the new wall. So I now basically have this concrete "slab" in front of the fireplace that is a tad *lower* than the plywood subfloor that would have butted up against it when it was in place (see attachment #1). My goal is to now construct a partition wall about 16-18" out from the original wall. I will frame this out so that we can do built-ins and a middle section for a bioethanol burner channel and a TV above it.

So, the existing concrete runs the entire length of the wall and is, of course, not level. So I was wondering if my best bet would be to use some form of "self-leveling" mix to bring this concrete slab thing to a level height, so that I could then attach the sole plate to it? I've never worked with that stuff before but I'm going to need some kind of level surface here correct? If it's a go that I should be leveling the surface with some self-leveling mix, do I bring it right up to the plywood subfloor? Or stop it 3/4" or 1" before it? Once the the leveling compound has cured/setup is it just like going into concrete/masonry at that point? I would use tapcons to attach the sole plate to the leveler-concrete underneath?

Also, on an install like this, what do you all recommend for the best way to make this wall "straight"? Just measure the sole plate a set amount of inches from the existing wall (the wall the heart was built-out from)? Use something like the 3-4-5 rule to the outer wall? Measure the top plate out a set amount of inches from the existing ceiling edge to make its line?

SLC certainly could be used for this, but it wasn't clear how deep you actually need it. Over a slab, slc can go from zero to as much as many inches (depending on the type). You can typically walk on it in a few hours, but you'd want to wait at least overnight to deal with a moisture sensitive wood like in a sole plate. While the stuff has great compressive strength, I'm not sure it would hold a TapCon really well, but if you used a long enough fastener to get into the old, slab below, it shouldn't be an issue. I wouldn't use a concrete nail, though. All slc installations require the use of the brand specific primer first, though.

A much less expensive option would be to do a bonded mudbed. This is a sand/concrete mix (typically something like 3-5:1 sand:concrete) that is bonded with a slurry of cement or thinset to the existing slab, mixed to a damp consistency such that it holds together when you squeeze a clump but doesn't drip water. You'd pack it down like wet beach sand, and you can screed it off to be level. Much cheaper than slc, but takes a bit more (not much!) skill. It has good compressive strength, but not much holding power for a fastener, so the same thing would apply, use one long enough to go into the existing slab.

Thanks Jim, I should had stated in the original post that I don't really want to add any height, just make the slab section "level" so that I can put down a sole plate for the new wall. I guess I don't really know why I posted the pic of the level-in-relation to the subfloor since the two don't really have a relationship?

In that case, I think I'd probably just shim the base plate of the new wall and not worry about it being level. There are some patching compounds you could use for those areas where you want new flooring to go if you want it level.

I agree with Jim It's not important that the area behind the partition is "level".
Shim the sill plate ,use construction adhesive ( liquid nails type ) to help secure to sill plate
and if you want you could secure it with tapcons. After you stick build on top of it and the
glue dries the plate won't go anywhere

Awesome. Thanks for all the replies! Final question: This room is the first floor of the house with a finished basement below it. Do I need to use PT lumber for the sole plate or can I just use a regular 2x4? And then do I need to use any particular kind of shim (like composite vs. wood)?

You should not need PT wood, and you'd be better off without it in this situation (IMHO). PT lumber, unless you can find KDAT stuff (kiln dried after treatment), is often so wet, that when it does eventually dry out, it can warp and twist into a pretzel. ANd, touching and cutting it gets messy - the sawdust isn't something you really want around, and the dampness doesn't help your tools, either. Plus, the extra moisture doesn't help. Since this has a basement underneath, I think you can safely treat it as if it were just any other interior room.

Thanks Jim. I think I'm about set in my approach now with the helpful info here. I think my last question is about the top plate.

The top plate is going to fall between joists so I'm going to have to put some blocking in. The issue up above is that the attic space is all blown-in cellulose. So if I cut it away to attach the top if the new wall directly to the new blocking (which I should be able to get installed from the top without messing with the drywall) I will have to cut away the drywall which could rain down a bunch of that crap (and it has that awesome tendency to collapse in in itself like a car falling backwards down a hill) and leave us with with an opening to the attic space. So my question is whether or not I can just attach the top plate to the blocking *through* the drywall? Or do I have to cut it out first and connect the top plate directly to the blocking?

Whoa. In what can only be labeled as super-dumb luck, I went into the attic just now and found the fish stick I put through the hole and it was *exactly* resting against the back of a joist! (Stud finder didn't turn up anything when I originally tried it so when the pilot but popped through I just assumed I was in between the joists) So I should be cool to just nail this top plate to the joist-no blocking!

So I guess is still need to know if I can just nail the top plate to the joist without removing the drywall? Or if I have to cut out the channel first? I'm hoping the prior because the latter would require something to be done to the drywall correct? I mean it would essence be hanging like 15" unattached from the screws in the previous joist?